The present invention relates to a variable displacement type swash plate compressor, and, more particularly, to a variable displacement type swash plate compressor capable of reducing power consumption of a compressor when an air-conditioning system is off and a displacement control valve for use in the compressor.
Typically, a compressor for compressing refrigerant gas is incorporated in a cooling circuit for a vehicle air-conditioning system. Such compressors are generally driven by the vehicle's engine and are often coupled to the engine by an electromagnetic clutch mechanism. The electromagnetic clutch connects the compressor to the engine only when a cooling load exists. Providing a compressor with the electromagnetic clutch mechanism however increases the total weight and the manufacturing cost, and the clutch draws power from the engine.
As a solution to those problems, a clutchless compressor has been proposed that directly connects the compressor to the engine and transmits power to the compressor whenever the engine is running. Recently, variable displacement type swash plate compressors have been considered suitable for such clutchless systems. Variable displacement type swash plate compressors are good at variably controlling the compression performance (discharge displacement) according to a variation in cooling load, either automatically or by means of an external control unit. However, they continuously apply a load to the engine.
As long as the cooling load is high and continuous, a clutchless, variable displacement type swash plate compressor works well. However, there is a need to reduce the load applied to the engine by the compressor when the cooling function is stopped in response to an external command, such as when a person in the vehicle turns off the air-conditioning switch.
In general, the discharge displacement of a variable displacement type swash plate compressor is controlled by adjustment of the piston stroke, which is accomplished by controlling the angle (inclination angle) of a swash plate with respect to the drive shaft by means of a displacement control valve. The inclination angle of the swash plate is controlled by controlling the internal pressure (Pc) of a crank chamber defined in the housing. Specifically, the internal pressure Pc of the crank chamber is increased to decrease the inclination angle, which reduces the discharge displacement. To tilt the swash plate in a direction that increases the inclination angle with such a structure, the swash plate must move toward the maximum inclination angle when the internal pressure Pc of the crank chamber falls. To return the swash plate to its maximum inclination angle, the minimum inclination angle should not be in the vicinity of 0.degree. (as measured with respect to a plane perpendicular to the drive shaft). That is, with the minimum inclination angle of the swash plate set near 0.degree., little or no compression takes place, and no compression reactive force large enough to regain the maximum inclination angle is produced. This makes it very difficult or impossible to return the swash plate back to the maximum inclination angle. It is therefore necessary to set the minimum inclination angle of the swash plate to about a range of +30.degree. to +50.degree., for example, so that there is some discharge from the compressor, even at the minimum inclination angle, which produces a small but significant compression reactive force. The compression reaction force contributes to increasing the inclination angle of the swash plate at the appropriate time. This permits the swash plate angle to increase in response to a reduction in the internal pressure Pc of the crank chamber, which is caused by the displacement control valve.
If a conventional variable displacement type swash plate compressor is designed as a clutchless type and is installed in a vehicle air-conditioning system, even when the start switch for the air-conditioner is turned off to set the inclination angle of the swash plate to the minimum inclination angle, the compressor continues operation with a minimum discharge displacement to continuously apply a compression reactive force to the swash plate. Thus, a small load is always applied to the vehicle engine. To reduce the load when the air-conditioning system is off, it is necessary to make the compression reactive force as low as possible by reducing the inclination angle of the swash plate as much as possible. If the compression reactive force is set too low, the swash plate cannot be inclined when there a need to increase the displacement. Since there is a compromise between reducing the power consumption under the minimum discharge displacement and using the compression reactive force to incline the swash plate to the maximum inclination angle, it is necessary to precisely adjust the minimum discharge displacement (or the minimum inclination angle) to satisfy both requirements. This is difficult to achieve in conventional variable displacement type swash plate compressors, which leads to increased manufacturing costs.